Alloys

Pure Metals Atom Arrangement

Pure metal is soft and not very strong. Atoms of pure metals have similar sizes and shapes and are arranged closely but there is still space between the atoms. These spaces allow the atoms to slide along one another easily when force is applied. This property causes pure metal to be ductile, that is, ability to be stretched into a wire.

When knocked or hammered, the metal atoms slide along one another to fill the spaces between them. This property makes pure metal to be malleable, that is, can be shaped into various shapes.

Definition

An alloy is a compound formed from a mixture of metals and other elements. The foreign atom, which is an impurity atom, may be atoms of other metals or non-metal such as carbon. The process of forming an alloy is called alloying.

Purposes

There are a few reasons why alloys are made and they are to;

• make the metal harder and stronger
• prevent corrosion of the metal
• improve the appearance of the metal so it looks more attractive

These are a few examples;

99% Iron + 1% Carbon = Steel

74% Iron + 18% Chromium + 8% Carbon = Stainless Steel

90% Copper + 10% Tin = Bronze

70% Copper + 30% Zinc = Brass

75% Copper + 25% Nickel = Copper Nickel

97% Tin +2% Copper + 1% Antimony = Pewter

93% Aluminium + 3% Copper + 3% Magnesium + 1% Manganese = Duralumin

Alloy's Properties

Impurity atoms which are mixed may be larger or smaller than atoms of pure metals. These fill the empty spaces between the pure metal atoms. Impurity atoms can prevent the layers of metal atoms from sliding along another easily. Due to this, an alloy is harder and stronger than pure metals.

source: www.zimbio.com/CarbonMonoxideAla...m/Metals

Comparing The Hardness

Aim : To study the hardness and strength of an alloy as compared to a pure metal.

Problem Statement : Is brass (copper alloy) harder then copper?

Hypothesis : Diameter of depression on brass is smaller than diameter of depression on pure copper.

Variables : Manipulated:Type of metal block

                Responding :Diameter of depression

                Constant     :Height of metal ball

Material : Copper block, brass block, metal block, metal ball (steel), cellophane tape, thread.

Apparatus : Metre rule, 1 kg weight, retort stand.

Apparatus to study the hardness and strength of an alloy
source: Longman Essential Chemistry SPM

Procedure :

1. A steel ball is stuck to a brass block using cellophane tape.
2. A weight of 1 kg is hung at a height of 50 cm from the top of the brass block.
3. The weight is released so that it falls on the metal ball.
4. The diameter of depression formed on top of the brass block is measured using a metre ruler and the measurement is recorded in Table 9.5
5. Steps 1 to 4 are repeated twice on different spots on the brass block to obtain an average of the diameter
6. The experiment is repeated using a pure copper block.

Results :

Block	Diameter of depression
	1	2	3	Average diameter (mm)
Brass
Copper

Analysis : 

1. The depression made on the brass block is smaller than the depression made on the pure copper block.
2. Alloy can be  defined operationally as a material which is harder because the depresssion produced is smaller.

Conclusion :

1. Alloys are harder than pure metals.
2. The hypothesis made is accepted.

Comparing The Rate Of Rusting

Aim : To study the rate of rusting of an alloy as compared to pure metals.

Problem Statement : Does pure iron rust faster than stainless steel nail.

Hypothesis : Iron rusts faster than stainless steel.

Variables : Manipulated:Type of nail

                Responding :Intensity and amount of blue colour

                Constant     :Duration of rusting

Materials : Iron nails, stainless steel nails, agar, potassium hexacyanoferrate(|||) solution.

Apparatus : Test tube, test tube rack.

Apparatus to study the rates of corrosion of iron and stainless steel nails
source: Longman Essential Chemistry SPM

Procedure :

1. Hot agar solution is prepared and poured into two test tubes.
2. A little potassium hexacyanoferrate(|||) solution is added.
3. Iron nails are placed in one test tube and stainless steel nails in the other.
4. The apparatus is left for several days.
5. A change in colour is observed in the test tubes.

Observation :

Experiment	Observation
Iron nails	Dark blue spots form
Stainless steel nails	No dark blue spots

Analysis :

1. If iron corrodes (rust), iron(||) ions form. Potassium hexacyanoferrate(|||) solution can detect the presence of iron(||) by forming a dark blue colour.
2. If the nails corrode (rust), dark blue spots form in the agar.
3. Dark blue spots form only in the test tube containing the iron nails, so we can conclude that iron corrodes but not stainless steel.
4. Alloy can be defined operationally as a material which is resistant to corrosion becauseno dark blue spots are produced.

Conclusion :

1. Iron corrodes (rust) faster than steel.
2. The hypothesis made is accepted.

Uses of Alloys

Steel

use = To make bridges, vehicles, building skeleton and train tracks.

Properties = Strong, hard, withstand corrosion.

Stainless Steel

Use = To make kitchen utensils such as spoons, fork pots, pans and knives.

Properties = Strong, withstands corrosion, shiny.

Bronze

Use = To make medals, statues and bells.

Properties = Strong, hard, withstands corrosion.

Brass

Use = To make keys, musical instruments and ornaments.

Properties = Strong, shiny.

Pewter

Use = To make ornamental items such as picture frames and trophies.

Properties = Withstands corrosion, smooth and shiny surface.

Duralumin

Use = To make the body of aeroplanes and racing bicycles.

Properties = Strong, light, withstands corrosion.

Copper Nickel

Use = To make coins.

Properties = Strong, shiny silver colour.

References

-Longman Essential Chemistry SPM